Researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators identified a new fossil oak species in northern Thailand. The newly identified species, Quercus liensis, provides crucial evidence that the region experienced a cooler, wetter subtropical climate during the Late Oligocene to Early Miocene epoch, about 25 million years ago.

The study was published in Geobios on August 6.

The fossil leaves, which are exceptionally well preserved, were discovered in the Li Basin in Lamphun Province. Dating to between 23 and 33 million years ago, the leaves are characterized by their distinct architecture, including an acuminate apex, serrated margin, and specific venation pattern. Microscopic analysis also revealed anomalocytic stomata and unicellular trichome bases preserved on the leaf epidermis.

After conducting detailed morphological comparisons with both living and fossil species, the researchers confirmed that this species is new to science. Its closest living relative, Quercus shennongii, is currently only found in the subtropical region of southeastern China. This biogeographic link hints at a once continuous vegetation corridor.

Together with palynological evidence, the study revealed that, during the late Oligocene to early Miocene, subtropical evergreen broad-leaved forests dominated by Fagaceae existed in northern Thailand. These forests were significantly different from modern tropical monsoon forests.

The researchers suggest that this dramatic shift in climate and vegetation was driven by a combination of powerful forces. The global cooling event that occurred during the Oligocene epoch, one of the most significant climate shifts in Earth's history, played a major role. Concurrently, the tectonic southward extrusion of the Southeast Asian landmass, which pushed the region into lower latitudes, contributed to a long-term change in temperature and rainfall patterns.

"Quercus liensis currently represents the southernmost fossil occurrence of its kind in the world. Its presence in Thailand is a clear indicator of a southward expansion of these subtropical plants into Southeast Asia during the Oligocene, a movement likely prompted by the same global cooling and paleogeographic shifts," said HUANG Jian of XTBG.

The study provides crucial evidence for the southward expansion of subtropical plants and helps researchers understand the long-term impacts of climate and tectonic changes in the region.

Fossil of Quercus liensis (Image by Napussawan Thongsangtum)